This invention relates generally to improved methods for controlling weeds in a crop of transgenic glyphosate-tolerant cotton plants by over-the-top, foliar application of a herbicidal glyphosate formulation. The present invention is further directed to herbicidal glyphosate compositions useful in practicing the weed control methods disclosed herein.
Cotton (i.e., Gossypium hirsutum) provides an ideal fiber for textile manufacture as well as oil for human consumption, feed for livestock and base chemicals for a variety of industrial products. Cotton production is well-established in the United States and many other areas of the world. As in other cultivated crops, weeds can cause significant yield losses and require careful management by the grower as they interfere through their competition for available resources including water, nutrients and light. In cotton, weeds can also impede harvest and have a negative economic impact on the grower by not only reducing cotton lint yields, but also lint quality. Weed control practices in cotton have included cultural, mechanical, biological and chemical methods. Among these, chemical weed control has been widely adopted along with the use of tillage (e.g., seed bed preparation, tillage) and cultural (e.g., crop rotation, field selection) methods.
N-(phosphonomethyl)glycine, known in the agricultural chemical art as glyphosate, is a highly effective and commercially important broad spectrum phytotoxicant useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. Glyphosate is used as a post-emergent herbicide to control the growth of a wide variety of annual and perennial grass and broadleaf weed species in cultivated crop lands, including cotton production, and is the active ingredient in the ROUNDUP family of herbicides available from Monsanto Company (Saint Louis, Mo.).
Glyphosate and salts thereof are conveniently applied in aqueous herbicidal formulations, usually containing one or more surfactants, to the foliar tissues (i.e., the leaves or other photosynthesizing organs) of the target plant. After application, the glyphosate is absorbed by the foliar tissues and translocated throughout the plant. Glyphosate noncompetitively blocks an important biochemical pathway that is common to virtually all plants. More specifically, glyphosate inhibits the shikimic acid pathway that leads to the biosynthesis of aromatic amino acids. Glyphosate inhibits the conversion of phosphoenolpyruvic acid and 3-phosphoshikimic acid to 5-enolpyruvyl-3-phosphoshikimic acid by inhibiting the enzyme 5-enolpyruvyl-3-phosphoshikimic acid synthase (EPSP synthase or EPSPS) found in plants.
Advances in genetic engineering have provided the requisite tools to transform cotton and other cultivated plants to contain foreign genes for improvement of certain agronomic traits and the quality of the product. One such trait of particular agronomic and environmental importance is herbicide tolerance, in particular, tolerance to glyphosate herbicide. Glyphosate-resistant or tolerant crop plants may reduce the need for tillage to control weeds, thereby effectively reducing soil erosion. Further, glyphosate-tolerant crop plants provide greater simplicity and flexibility in attaining adequate weed control.
Glyphosate-tolerant cotton can be produced, for example, by introducing into the genome of the plant, the capacity to express various native and variant plant or bacterial EPSPS enzymes that have a lower affinity for glyphosate and therefore retain their catalytic activity in the presence of glyphosate (See, for example, U.S. Pat. Nos. 5,633,435, 5,094,945, 4,535,060, 6,040,497 and 6,740,488). Glyphosate-tolerance has been introduced into cotton plants and is a successful product now widely used in cotton production. The current commercial ROUNDUP READY cotton event designated 1445 available from Monsanto Company provides excellent resistance to glyphosate. Glyphosate is typically applied over-the-top (OTT) of ROUNDUP READY cotton from emergence through the four leaf node stage of development (e.g., at rates of up to about 0.75 pounds glyphosate acid equivalent per acre (lb a.e./A or about 0.84 kg a.e./ha). ROUNDUP READY cotton varieties used in combination with ROUNDUP glyphosate herbicidal formulations have become the standard program for weed management in cotton production in the United States. The primary advantage to growers for using the ROUNDUP READY cotton system is that it allows simple and convenient application of glyphosate, a broad spectrum, post-emergence herbicide, to effectively control weeds and grasses with excellent crop safety and less dependence on pre-plant herbicide applications. Other benefits include a better fit into no-till and reduced tillage systems. ROUNDUP READY cotton has expanded the options for weed management and made the practice of weed control much easier, less expensive and more flexible. Growers have reported making fewer trips across fields to apply herbicides as well as making fewer cultivation trips, which conserves fuel and reduces soil erosion. Glyphosate-tolerant cotton, therefore, decreases the environmental risks posed by herbicides while at the same time increasing the efficacy of necessary chemical weed control.
Although widely accepted as a standard in cotton production, ROUNDUP READY cotton varieties do however impose some limitations on the grower. ROUNDUP READY cotton varieties possess reproductive bodies that are susceptible to glyphosate-mediated injury that may, in some cases, result in delayed maturity or reproductive injury as measured by flower pollen shed, flower drop, boll drop, and/or lint yield loss. Accordingly, in order to avoid or minimize such reproductive injury, over-the-top applications of glyphosate herbicides to cotton plants grown from seed of ROUNDUP READY cotton event 1445 and progeny thereof are generally discontinued from the fifth leaf node stage and beyond (e.g., through layby) and instead glyphosate is usually applied as a post-directed spray between the crop rows during this period of growth in order to minimize contact with the cotton plants. Directed herbicide application requires specialized equipment that is often susceptible to misapplication, must be operated at lower speeds and requires a greater number of trips per acre compared to broadcast applicators.
It is believed that the lack of reproductive glyphosate tolerance that limits later stage foliar application of glyphosate to cotton plants corresponding to ROUNDUP READY cotton event designated 1445 is the result of insufficient CP4 EPSPS expression in critical tissues, higher sensitivity of these tissues to glyphosate, and accumulation of high amounts of glyphosate in those strong sink tissues. Recently, as disclosed in International Publication No. WO 2004/072235, Monsanto Company has developed a new glyphosate-tolerant cotton event (designated MON 88913, to be commercially named ROUNDUP READY FLEX cotton and having seed deposited with American Type Culture Collection with Accession No. PTA-4854) to provide cotton growers with an improved product for management of economically damaging weeds. ROUNDUP READY FLEX cotton provides an increased margin of fruit retention and crop safety, due to increased tolerance to glyphosate in reproductive tissues. This allows for an expanded window for over-the-top ground application of glyphosate agricultural herbicides (e.g., at rates of up to about 1.125 pounds glyphosate acid equivalent per acre (lb a.e./A or about 1.26 kg a.e./ha) extending from cotton emergence up to layby, the critical timing for weed control in cotton. Through these enhanced treatment opportunities, the grower can more effectively manage weed control in cotton using over-the-top herbicide applications as compared to post-directed or hooded-sprayer applications.
Despite the widely-recognized advantages in weed control provided by ROUNDUP READY and ROUNDUP READY FLEX cotton, it has been observed that these transgenic cotton varieties, under certain environmental conditions, exhibit a susceptibility to leaf tissue necrosis following over-the-top application of glyphosate herbicides. In the case of over-the-top application of glyphosate herbicides to ROUNDUP READY FLEX cotton at later stages of plant development, the appearance of necrotic lesions on the treated cotton plants may appear to be more pronounced due to the more fully developed canopy and greater available leaf area. This phenomenon is rare and such leaf injury, if it is encountered at all, is generally limited with little or no further expression of injury and the cotton plants recover with essentially no yield loss or deleterious effect on fertility under standard or recommended ROUNDUP treatment protocols. In particular, the cotton apical growing point and subsequent leaves appear unaffected.
Accordingly, there exists a need for methods and herbicidal glyphosate formulations useful for over-the-top, foliar application to transgenic glyphosate-tolerant cotton plants that are effective in weed control and consistently avoid inducing significant leaf necrosis in the treated plants in the variable environmental conditions that may be encountered during the growing season.